Crucible surface ionization source



Jan. 11, 1966 INVENTORS harles SrfeyezrS jgrdrewz A flazlzr ss jliorzy y United States Patent CRUCIBLE SURFACE 1ONlZATION SOURCE Charles M. Stevens, Naperville, and Andrew L. Harlrness,

Elmhurst, Ill., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 30, 1963, Ser. No. 312,813 6 Claims. (Cl. 315-111) The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The present invention relates to an improved ion source for a mass spectrometer. In particular it is a high ethciency source producing a substantially larger proportion of ionized particles from the charged sample material. It has particular advantages when the sample is extremely small, particularly when the sample is less than 1 milligram. This is particularly important when the sample contains trans-plutonium elements.

The volatilization temperature is usually substantially below the temperature required for efiicient surface ionization. With plutonium a temperature of 2000 to 2400 K. sufiices for volatilization, but 2800 to 3000 K. is required for the most efficient ionization.

Filament ionization sources are known in the art wherein the sample is placed on a filament which is heated by an electric current. With a simple single filament surface ionization source, by the time the temperature is sufiicient for ionization, substantially all of the material has volatilized away, unless the sample is comparatively large. In order to overcome this difliculty, a two filament system has been tried, as shown in US. Patent No. 2,733,343. In this case, the lower filament is operated at volatilization temperature and the upper one at the higher ionization temperature. The difficulty with this arangernent is the small ionization area offered to the escaping vapor, so that only this percentage of the vaporized molecules is ionized.

In the present invention the vaporization is accomplished in a closed container maintained at the proper temperature. The vapor is constrained to a narrow passage in escaping and this passage is maintained at the ionization temperature of 2800 to 3000 K., improving the total overall ionization of the vapor by a substantial factor.

The invention is best described by reference to the drawing. The single figure is a partially schematic scaled drawing showing an embodiment of the invention that proved to be quite effective.

In the single figure, crucible 2 is a rod of tungsten /8" in diameter about long, with a A sample containing cavity 4 bored at its upper end to a depth of about /a inch. A counterbore 6 extends down about inch. Crucible 2 fits into crucible support 8, a /s bar of tantalum bored to receive and support it. Ionizing tube 10 is a tungsten tube O.D., .040" ID. about /2" long. The lower end of tube 10 is enlarged by two ridges extending to a diameter of 0.093", and so spaced as to fit moderately snugly into counterbore 6 of crucible 2. The crucible 2, support 8 and ionizing tube 10 are maintained at a positive potential of 15,000 volts with respect to ground by lead wires (not shown).

The sample to be ionized is loaded onto a short length of tungsten wire or ribbon and placed in the sample containing cavity 4 of the crucible 2. The ionizing tube 10 is then fitted into the counterbore 6 of the crucible.

A first circular ribbon filament 12 is spaced around crucible 2 and connected to a first variable electric current heating circuit (not shown). A second circular ribbon filament 14 is spaced around the upper portion of ionizing tube 10 and connected to a second variable electric cur- "ice rent heating circuit (not shown). A tantalum sheet radiation shield 16 is positioned so as to cover the filaments 12 and 14, so as to reflect back and retain the heat produc'ed in the filaments. Both filaments and shield are maintained at a negative potential of 1500 volts relative to crucible 2 by a voltage source (not shown). Heating is produced by the bombardment of the crucible by electrons thermally produced in filaments 12 and 14 and accelerated by the 1500 volt difference in potential. Temperature ditierences are maintained by controlling the electric current heating of filaments 12 and 14; the relatively loose fits between crucible 2 and support 8 and between ionizing tube 10 and crucible 2 limit the heat transfer therebetween.

Ordinarily the current to filament 12 is so adjusted to maintain a temperature of 2000 to 2400 K. in the adjacent portion of crucible 2. The current of filament 14 is adjusted to maintain a temperature of 2800 to 3000 K. in the adjacent portion of ionizing tube 10.

Focusing of the resulting ion beam is accomplished by a first cylindrical electrode 18 and a second cylindrical electrode 20, positioned above the ionizing tube 10 coaxially. The first electrode 18 is a cylinder of tantalum O.D., long, about 10 mils thick, supported by a tantalum bar connected to an insulating support (not shown). The second electrode 20 is a cylinder of stainless steel /s" O.D., long, 20 mils thick, supported in the same way. Electrode 18 is maintained at a negative potential of 1000 volts with respect to crucible 2, while electrode 20 is maintained at 1200 volts negative relative to crucible 2. Stainless steel is satisfactory for electrode 20 as it is relatively remote from the heated zone.

Above electrode 20, two radial lens elements 22 are positioned. Each element 22 consists of a sheet of stainless steel .010" thick, /2 inch wide and 4 inch long, bent to a right angle, positioned so that two surfaces extend downward bounding the emergent ions. There is a mechanism (not shown) to position the elements so as to center them and adjust the gap therebetween. These lens elements are maintained at a negative potential of about 8000 volts relative to crucible 2.

Above the radial lens elements 22 is positioned a pair of collimating elements 24. These collimating elements are rectangular stainless steel plates with one edge beveled. The beveled edges are positioned symmetrically about the central plane of the radial lens elements 22, to form a collimating slit 26. There is a mechanism (not shown) to adjust and center this slit.

A pair of beam centering elements 28 are positioned above the collimating elements 24- and share the same central plane. Each beam centering element is a rectangular sheet of .010" stainless steel, bent upward at a right angle.

Above the beam centering elements 28, is positioned a pair of adjustable entrance slit elements 30, with beveled edges. These elements 30 are also adjustable for centering and adjusting the slit formed therebetween.

The entrance slit elements 30, the beam centering elements 28 and the collimating slit elements 24 are all maintained at a negative potential of 15,000 volts relative to the crucible.

The entire apparatus is centered to enter the conventional analyzer section (not shown) of a conventional spectrometer.

The cylindrical electrodes 18 and 20 have an advantage over conventional slit plate in that they will intercept much less of the radiated heat by virtue of their smaller cross sectional area. A further advantage is that focusing is radial rather than unidirectional.

In test, it was found that a heating current of 50 milliamperes was sufficient to heat the adjacent sample portion of crucible 2 to a temperature of about 2400 K. and thereby vaporize the sample. A heating current of Example Tests were made using samples of 5 10 grams of uranium and l0 grams of plutonium. Two uranium samples and two plutonium samples wereionized using the apparatus of the present invention. Two corresponding uranium and plutonium samples were ionized using the standard double filament ionization source.

The detection instrument had a 12 inch radius, 60 sector tube. The magnet sector was 44 to give partial axial focusing. Accelerating voltage was about 14,500 volts.

Measurements were made by placing a grid directly in front of the collector slit to intercept a fixed fraction of all the isotopes of the element being measured. The current to the grid was integrated to obtain a measurement of the fixed fraction of the total ions reaching the collector. From this, the total ions which would have reached the collector in the absence of the grid was obtained.

The basic data calculated for each run was the fraction of ions reaching the collector to total atoms initially present in the sample. Table I shows comparative results for the double filament apparatus and the apparatus of the present invention.

It is readily discernible that the eificiency of the apparatus of the present invention is much superior to the double filament apparatus. As indicated in the last column of the table, the two uranium tests showed improvement by factors of 32 and 12, respectively, and the two plutonium tests showed improvement by factors of 35 and 72 respectively. It is noteworthy that in one plutonium run using the ion source of the present invention almost one percent of the total plutonium in the sample was collected.

Since rhenium has a higher work function than tungsten and almost as high a melting point as tungsten, it is considered an equivalent material for use for the crucible 2 and the ionizing tube 10.

It will be understood that the invention is not to be limited to the details of the example or other details given 4 herein, but that it may be modified within the scope of the appended claims,

The embodiments of the invention in which an exclu sive property or privilege is claimed are defined as follows:

1. An ion source comprising an elongated refractory metal crucible having a sample containing cavity at one end thereof; means for heating said one end of said crucible to approximately 2400 K., whereby the contained sample is vaporized; a small diameter refractory metal open-ended ionization tube having a length to'inside diameter ratio of at least about 12, one end of said tube extending partially into said cavity s'o'as 'to be supported by said crucible and to have its interior volume in communication with said cavity; and means for heating the other end of said tube to a temperature of at least 2800" K.

2. The ource of claim 1 where the heating means are electric current heated filaments spaced around the crucible and the tube and maintained at a negative potential of 1500 volts with respect to said crucible.

3. The source of claim 1 in which the crucible is a rod A2." in diameter, centrally drilled A for a depth of about /8 and counterbored for 56 to a diameter and the tube is about /2" long, outside diameter and 0.040" inside diameter, with the bottom slightly enlarged such as to engage the shoulder in the crucible.

4. The source of claim 3 where the refractory metal forming the crucible and ionization tube is selected from the class consisting of tungsten and rhenium.

5. An ion source, as claimed in claim 2, and further including a first cylindrical electrode consisting of a refractory metal cylindrical tube having outside diameter and length about triple the outside diameter of said tube positioned coaxially adjacent the said other end of said tube and maintained at a negative potential of about 1000 volts with respect to said tube, and a second cylindrical electrode, about double the length and outside diameter of said first electrode, positioned coaxially adjacent said first electrode and maintained at a negative potential of about 1200 volts with respect to said tube.

6. The ion source, as claimed in claim 5, and further including a pair of half slits spaced from and aligned With said second electrode and maintained at about 8000 volts negative potential relative to said tube, and a system of adjustable collimating slits spaced from and aligned with said pair of half slits and maintained at a negative potential of about 15,000 volts with respect to said tube.

References Cited by the Examiner FOREIGN PATENTS 1,289,265 2/1962 France.

GEORGE N. WESTBY, Primary Examiner. S. D. SCHLOSSER, Assistant Examiner. 

1. AN ION SOURCE COMPRISING AN ELONGATED REFRACTORY METAL CRUCIBLE HAVING A SAMPLE CONTAINING CAVITY AT ONE END THEREOF; MEANS FOR HEATING SAID ONE END OF SAID CRUCIABLE TO APPROXIMATELY 2400* K., WHEREBY THE CONTAINED SAMPLE IS VAPORIZED; A SMALL DIAMETER REFRACTORY METAL OPEN-ENDED IONIZATION TUBE HAVING A LENGTH TO INSIDE DIAMETER RATIO OF AT LEAST ABOUT 12, ONE END OF SAID TUBE EXTENDING PARTIALLY INTO SAID CAVITY SO AS TO BE SUPPORTED BY SAID CRUCIBLE AND TO HAVE ITS INTERIOR VOLUME IN COMMUNICATION WITH SAID CAVITY; AND MEANS FOR HEATING THE 